Area Forecast Discussion
Issued by NWS Riverton, WY

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073
FXUS65 KRIW 020545
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1045 PM MST MON FEB 1 2016

.SHORT TERM...TONIGHT THROUGH TUESDAY

THE MAIN LOW THAT IS RESPONSIBLE FOR THE SNOW THAT IS CURRENTLY
FALLING OVER CENTRAL...SOUTHERN AND EASTERN WYOMING IS CENTERED
OVER SOUTHWEST WY ATTM. THIS CIRCULATION WILL BE TRACKING EAST
ACROSS CO AND KS DURING THE COURSE OF THE NIGHT TONIGHT.
MEANWHILE...ISENTROPIC LIFT AHEAD OF AND NORTH OF THE STORM
CENTER HAS EXTENDED AS FAR NORTH AS RIVERTON AND ON UP INTO
JOHNSON COUNTY AND HAS PLAYED OUT IN A BROAD AREA OF STEADY LIGHT
SNOWFALL. IN MOST OF THE AFOREMENTIONED AREAS...1 TO 2 INCHES OF
SNOWFALL HAS ACCUMULATED SO FAR WITH CONSIDERABLE DRIFTING.
HOWEVER...AREAS FARTHEST TO THE SOUTHEAST LIKE CPR...CLOSER TO THE
TRACK OF THE MORE POTENT DYNAMICS/QG FORCING/LOW LEVEL UPSLOPE...ANYWHERE
FROM 6 TO 10 INCHES OF SNOW HAS ALREADY FALLEN. WE KEPT THE
ADVISORIES GOING ACROSS PORTIONS OF SOUTHERN AND CENTRAL WY BUT
EXTENDED THEM NORTH TO INCLUDE RIVERTON/WIND RIVER BASIN AND NORTH
INTO SOUTHERN JOHNSON COUNTY TO ACCOUNT FOR BLOWING AND DRIFTING
EVEN THOUGH MANY OF THESE AREAS WILL SEE SUB ADVISORY AMNTS.
THERE HAS BEEN MORE OF AN EASTERLY COMPONENT TO THE CIRCULATION AT
THE MID LEVELS OVER OUR CWA THAN THE MORE TRADITIONAL DEEPER NNE
COMPONENT ASCT WITH STRONGER UPSLOPE EVENTS OVER THE LANDER
FOOTHILLS AND THE ADJACENT WIND RIVER MTNS THUS FAR WHICH HAS
ACCOUNTED FOR THE EAST TO WEST SNOWBANDS THAT OCCURRED OVER THE
WIND RIVER BASIN EARLIER TODAY...BUT AS THE LOW CONTINUES ON ITS
EASTERLY TRACK...UPSLOPE WILL DEEPEN FOR CONTINUED SNOWFALL OVER
LANDER. HOWEVER...THE NORTHERLY TRAJECTORY WILL ENHANCE THE OWL
CREEK MTN SNOW SHADOW AT AND NORTH OF RIVERTON BY MID EVENING FOR
SNOW TO DIMINISH THERE. SHOSHONI STILL HAS NOT RECEIVED ANY
ACCUMULATION SO FAR. WE UPGRADED THE ADVISORY TO A WARNING FOR
NATRONA COUNTY/CASPER MTN FOR HIGHER SNOW AMOUNTS THAN
ANTICIPATED. THINKING NOW IS THAT CASPER COULD WIND UP WITH 6 TO 9
INCHES ACROSS CASPER AND 9 TO 16 INCHES ON CASPER MTN. THE GFS/NAM
APPEAR TO BE OVER PERFORMING IN THE RIVERTON AREA AND
UNDER PERFORMING IN THE CPR AREA FOR QPF. EXCEPT FOR THE HEAVY
SNOWFALL IN THE CPR AREA...THE LOCAL WRF APPEARS TO HAVE THE
DISTRIBUTION OF THE SNOW NAILED DOWN FOR THE REST OF THE NIGHT AS
FAR AS UPSLOPE/DOWNSLOPE COMPONENTS ARE CONCERNED FOR THE
REMAINDER OF THE SNOW EVENT TONIGHT. KEPT THE HILITES GOING
THROUGH 9 AM TUE TO ACCOUNT FOR FRESHLY FALLEN FLUFF THAT COULD
STILL BE BLOWN AROUND DURING THE MORNING COMMUTE AS WE REMAIN
UNDER THE CYCLONIC CIRCULATION OF THE STRENGTHENING LOW OVER
KANSAS. TEMPS WILL BE SEASONABLY COLD TUE BEHIND THIS SYSTEM. THEN
OUR ATTENTION QUICKLY SHIFTS TO THE FAR WEST WY AS A SHORTWAVE
TRACKS SOUTHEAST ALONG THE BACK SIDE OF THE EXITING MAIN TROUGH
ALONG WITH ITS DIFLUENT FLOW FOR AN INCREASING CHC OF LIGHT SNOW
IN THE FAR WEST TUESDAY AFTERNOON.

.LONG TERM...TUESDAY NIGHT THROUGH MONDAY

THIS PERIOD STARTS OUT WITH A COLD SEWD MOVING SHORTWAVE THAT
WILL HAVE TO BE WATCHED FOR SNOW SHOWERS ALONG ITS PATH AS LAPSE
RATES ARE STEEP TUESDAY NIGHT (>7.5C/KM IN THE H7-5 LAYER) AT -35
TO -37C H5 AIR DROPS SE WITH IT. HAVE ADDED IN SOME ISOLATED TO
SCT SNOW SHOWERS ACROSS THE ERN AREAS THAT ARE FAVORABLE IN THIS
PATTERN. WILL HAVE TO WATCH THIS AREA FROM RIW TO CASPER. HIT THE
NATRONA AREA THE HARDEST FOR NOW BUT WE`LL JUST NEED TO SEE HOW IT
LOOKS AS IT GETS CLOSER AS TO HOW MUCH POTENTIAL IT HAS FOR SOME
PASSING/TRAINING SNOW SHOWERS COMING OFF THE BIGHORNS AND POSSIBLY
FURTHER WEST TO THE BRIDGERS OR EVEN OWL CREEKS. QUICK...WEAK WARM
ADVECTION SHORTWAVE LIFTS EWD ACROSS THE WEST ON WEDNESDAY FOR
SOME VERY LIGHT SNOW OR FLURRIES IN THE HIGH COUNTRY. GAP FLOW
STARTS TO DEVELOP A LITTLE TUESDAY NIGHT AND THEN INCREASES WED
NGT AHEAD OF OUR THU SPLITTING TROUGH. THERMAL GRADIENT LOOKS
FAIRLY GOOD NORTH OF CODY WITH UP TO 15C AT H7. WRF IS ALSO
SHOWING SOME MTN WAVE ACTION WITH POTENTIAL UPSTREAM MTN TOP
INVERSION DEVELOPING WED NGT. WORTH KEEPING AN EYE ON. BEEFED UP
THE WIND FOR STARTERS. NEXT TROUGH ON THURSDAY LOOKS TO SPLIT WITH
MOST OF THE ENERGY GOING SOUTH. GFS HAS MORE ENERGY IN THE NRN
PART WITH POTENTIAL FOR SOME SNOW SHOWERS IN THE NERN ZONES AT
LEAST. THE NAM AT 84 HOURS HAS EVEN MORE ENERGY COMING THROUGH IN
THE NRN HALF OF THE TROUGH. GOING FORECAST IS A BLEND WITH SOME
SMALL POPS IN THE NORTH AND EAST AND ACROSS THE SOUTH WITH THE
HIGHER POPS IN THE WEST. BRIEF RIDGING FRIDAY INTO EARLY SATURDAY
BEFORE OUR NEXT SYSTEM MOVES IN. RIDGE BUILDS QUICKLY ALONG THE
COAST ALLOWING A FAIRLY GOOD COLD FRONT TO DIVE S-SE ACROSS THE
AREA LATER SATURDAY INTO SATURDAY NIGHT WITH SOME STRONG N-NW WIND
AND A PERIOD OF FRONTAL SNOW. LOOKS LIKE A QUICK HITTER BUT CHILLY
SUNDAY INTO MONDAY WITH EURO HOLDING THE COLDER AIR IN ALOFT
THROUGH MONDAY. OVERALL ECMWF IS COLDER BY AROUND 5C. -35 TO -38C
AT H5 SUNDAY INTO MONDAY MORNING VS A FURTHER NE -30 TO -35C ON
THE GFS. THIS SYSTEM IS CURRENTLY OUT BETWEEN THE WRN END OF THE
ALEUTIANS AND KAMCHAKTA WITH -46C AIR AT H5. IT WILL WARM SOME AS
IT TRACKS ACROSS THE OCEAN BUT AT A FAIRLY HIGH LATITUDE. PROBABLY
WON`T BE QUITE AS COLD AS THE ECMWF BUT A LARGER AREA OF COLD AIR
THAN THE GFS. COLD...NONETHELESS...LATE IN THE WEEKEND THROUGH
MONDAY WITH A LITTLE SNOW POTENTIAL WITH FROPA AND A FEW SNOW
SHOWERS BEHIND.

&&

.AVIATION.../06Z ISSUANCE/

WIDESPREAD IFR/MVFR CONDITIONS ARE EXPECTED THROUGH 12Z...GENERALLY
EAST AND SOUTH OF KRKS-KRIW-KBYG LINE. THE SNOW IS EXPECTED TO
GRADUALLY DECREASE IN COVERAGE DURING THE MORNING...WITH TERMINALS
BECOMING MAINLY VFR BY NOON...AS THE WINTER STORM CONTINUES TO PULL
AWAY FROM THE AREA. A WEAK SYSTEM IN THE NORTHWEST FLOW IN WAKE OF
THE MAIN SYSTEM COULD RESULT IN ISOLATED TO LOCALLY SCATTERED SNOW
SHOWERS LATE THIS AFTERNOON AND INTO TUESDAY NIGHT...AND WILL HAVE
VCSH TO ACCOUNT FOR THIS. WINDS WILL CONTINUE TO WEAKEN FOR THE
REMAINDER OF TONIGHT...AND BE MAINLY LIGHT DURING THE DAY TUESDAY.
HOWEVER BREEZY WEST TO NORTHWEST WINDS ARE EXPECTED TO DEVELOP
TUESDAY NIGHT OVER SWEETWATER...UPPER GREEN RIVER BASIN...AND THE
DUBOIS AREA. THIS WIND COULD EVEN IMPACT THE KRIW TERMINAL.

&&

.FIRE WEATHER...

PERIODS OF SNOW AND GUSTY WINDS ACROSS THE FORECAST AREA WILL RESULT
IN FAIR TO GOOD SMOKE DISPERSION TONIGHT...ESPECIALLY ACROSS
CASPER AND RAWLINS DISPATCH. MIXING HEIGHTS LOWER A BIT AFTER
TODAY BUT VARIABLE CLOUD COVER AND OCCASIONAL BREEZES WILL KEEP
SMOKE DISPERSION FROM BOTTOMING OUT ACROSS THE BOARD...THOUGH
THERE WILL BE SOME INVERSIONS RETURNING TO VALLEYS AND BASINS
AFTER TONIGHT...WITH SOME IMPROVEMENT DURING THE DAY. THERE WILL
BE A FEW SHOTS AT SOME MORE SNOW AS THE WEEK MOVES ALONG BUT
NOTHING ORGANIZED OR WIDESPREAD ON THE HORIZON. A STRONG RIDGE
COULD BE BUILDING IN NEXT WEEK...WITH DRIER AND CALMER WEATHER
POSSIBLE...IN ADDITION TO SOME STRONG INVERSIONS IN ANY SNOW
COVERED VALLEYS OR BASINS.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WARNING UNTIL 9 AM MST TUESDAY FOR WYZ020-022.

WINTER WEATHER ADVISORY UNTIL 9 AM MST TUESDAY FOR WYZ011-015-
017>019-027>030.

&&

$$

SHORT TERM...LIPSON
LONG TERM...SKRBAC
AVIATION...WM
FIRE WEATHER...LIPSON



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